Environmental awareness is growing in the U.S. and around the world leading to increasing public and regulatory pressures to reduce pollutant emissions from boilers, incinerators, and furnaces. One pollutant of particular concern is NOx (by which is meant oxides of nitrogen such as but not limited to NO, NO2, NO3, N2O, N2O3, N2O4, N3O4, and mixtures thereof), which has been implicated in acid rain, ground level ozone, and fine particulate formation.
A number of technologies are available to reduce NOx emissions. These technologies can be divided into two major classes, primary and secondary. Primary technologies minimize or prevent NOx formation in the combustion zone by controlling the combustion process. Secondary technologies use chemicals to reduce NOx formed in the combustion zone to molecular nitrogen. The current invention is a primary control technology.
In primary control technologies, different combustion strategies are used to control so called “thermal NOx” and “fuel NOx”. Thermal NOx is formed by oxidation of nitrogen molecules, N2, primarily in combustion air at high temperature. It is the main source of NOx emissions from natural gas and light oils that do not contain chemically bound nitrogen species. The main control strategy to reduce thermal NOx is to reduce peak flame temperature. Fuel NOx is formed by the oxidation of nitrogenous species contained in fuel and is the main source of NOx emissions from combustion of coal and heavy oil. The current invention relates to improved combustion methods to control fuel NOx emission.
The primary control technology for fuel NOx is commonly called staged combustion in which mixing between the combustion air and fuel is carefully controlled to minimize NOx formation. The formation of NOx from fuel nitrogen is based on a competition between the formation of NOx and the formation of N2 from the nitrogenous species in the fuel volatiles and char nitrogen. Oxygen rich conditions drive the competition towards NOx formation. Fuel rich conditions drive the reactions to form N2. Staged combustion takes advantage of this phenomenon by carefully controlling the mixing of air and fuel to form a fuel rich region to prevent NOx formation. To reduce NOx emissions, the fuel rich region must be hot enough to drive the NOx reduction kinetics. However, sufficient heat has to be transferred from the fuel rich first stage to the furnace heat load in order to prevent thermal NOx formation in the second stage.
A conventional low NOx burner (LNB) includes a fuel rich first zone, near the feed orifice, which is mainly controlled by mixing and combustion of fuel and primary air, and to some extent, additional secondary or tertiary air mixed in this zone. For combustion of pulverized coal the primary air is used to transport the coal particles.
In a second zone, the remainder of the secondary air and any tertiary air mix with the unburned fuel and products of partial combustion from the first stage and complete the combustion. An important process requirement for staged combustion is to transfer a sufficient amount of heat from the fuel rich first stage to the furnace heat load to cool down the combustion products from the first stage. Lower second stage temperature helps to reduce the conversion of remaining nitrogenous compounds to NOx and also to prevent thermal NOx formation in the second stage.
In an aerodynamically staged LNB, all of the combustion air is introduced from the same burner port or adjacent to the burner port. The most common configuration of a low NOx coal burner is to have a series of annular passages for coal/primary air, secondary air and tertiary air. The central passage is often used for oil gun or for natural gas for start up heating. Secondary and tertiary air flows are equipped with swirl generators to impart swirling flows to create a recirculation zone for flame stability. Air velocities and swirl are adjusted to create a relatively large fuel rich first zone along the axis of the burner, followed by relatively gradual mixing of secondary and tertiary air along the length of the furnace. Since sufficient air velocities must be provided to mix fuel and air within the furnace space to complete combustion, it is difficult to create a very large fuel rich zone to provide a long enough residence time for maximum NOx reduction.
Although the LNB is a fairly inexpensive way to reduce NOx and many advancements have been made in the burner design, currently available versions are not yet capable to reach the emissions limits in pending regulations of 0.15 lb (as NO2) per MMBtu of coal fired for utility boilers.
Those skilled in the art have overcome the limitations of an aerodynamically staged LNB by a globally staged combustion arrangement using “over fire air” (OFA). OFA is injected separately from a burner or a group of burners to provide a large fuel rich primary combustion zone (PCZ) and a burnout zone (BOZ) where combustion is completed by mixing OFA and unburned fuel and the products of partial combustion from the PCZ. Typically the OFA ports are separated at least one burner port diameter from the closest burner and several burner port diameters from the furthermost burner. Although the fuel and air mixing and the local stoichiometric conditions near the burner port of an individual burner are similar to those without OFA, a large fuel rich PCZ is formed outside the combustion air mixing zone near the burner. Due to the physical separation of the OFA injection ports, the residence time in the fuel rich PCZ is much longer than that typically obtained in the fuel rich first zone of an aerodynamically staged burner. The combination of LNB's and OFA ports has enabled further reduction in NOx emissions.
S. J. Johnson and R. J. Yang (“Interpretation of Small and Intermediate Scale test Results from a Low NOx Combustion System”, International Flame Research Foundation Advanced Combustion Technology Meeting, Noordwijkerhout, Holland, May 12-14, 1980) found that NOx emissions from the coal fired two stage combustion process is strongly dependant on the temperature of the second stage. Their experimental results conducted in small and intermediate scale test furnaces indicated that NOx emission is reduced by about 16% for every 100° F. drop in the theoretical second stage temperature.
Flue gas recirculation (FGR) to the second stage was one of the methods tested to reduce the second stage temperature. FGR can provide a large amount of cooling in the combustion chamber. However, the volume of the flue gas is increased by FGR, which causes a higher pressure drop and a greater heat transfer in the convective section and limits the maximum amount of FGR allowed for a give boiler. Furthermore, handling recycled flue gas is very maintenance intensive. Depending on where the flue gas is extracted from the system, it may contain fly ash and/or sulfuric acid vapors which would make it corrosive to recycling equipment. In addition, it will have residual levels of heat and typically low pressures which require larger pipes to handle the flow rates and may have higher than desired oxygen levels depending on the infiltration of air into the flue system and the overall stoichiometry of the boiler operation. This latter condition can also require greater volumes of flue gas to be required if diluting air to a certain oxygen level is required. The dirty nature of the gas also makes measurement and control difficult; and when systems break down, they are expensive to fix so repairs are not always done. In addition, any off-spec operation of the boiler (e.g. high CO levels) can bring these undesirable contaminants back into the boiler house, creating operator safety issues should leaks develop or process safety issues should combustible gases be present in the recycled stream. In addition, the composition of the flue gas recycled will depend on boiler operating conditions and can vary during boiler transients or off-spec operating conditions.
Low NOx burners and over fire air represent a fairly mature technology and as such are discussed widely throughout the patent and archival literature. Many ideas have been proposed to enhance the effectiveness of LNB's and OFA while minimizing detrimental impacts such as poor flame stability and increased carbon in the ash. Of these ideas two are particularly relevant: preheating the air to the first stage, and converting the combustor to oxy-fuel firing.
Both air preheat and oxy-fuel combustion enhance the effectiveness of staged combustion for fuel NOx reduction by increasing the temperature in the primary combustion zone without increasing the stoichiometric ratio. Oxy-fuel combustion offers the additional advantage of longer residence times in the fuel rich region, due to lower gas flows, which has been shown to reduce NOx emissions. As discussed above, staged combustion uses a fuel rich stage to promote the formation of N2 rather than NOx. Since the reactions to form N2 are kinetically controlled, both the temperature and the hydrocarbon radical concentration are critical to reducing NOx formation. For example, if the temperature is high and the radical concentration is low, such as under unstaged or mildly staged conditions, NOx formation is increased. When the radical concentration is high but the temperature is low, such as under deeply staged conditions, the conversion of intermediate species such as HCN to N2 is retarded. When air is added to complete burnout, the intermediates oxidize to form NOx, therefore the net NOx formation is increased.
Sarofim et al. “Strategies for Controlling Nitrogen Oxide Emissions During Combustion of Nitrogen bearing fuels”, 69th Annual Meeting of the AIChE, Chicago, Ill., November 1976, and others have suggested that the first stage kinetics can be enhanced by preheating the combustion air to fairly high temperatures. Alternately Kobayashi et al. (“NOx Emission Characteristics of Industrial Burners and Control Methods Under Oxygen-Enriched Combustion Conditions”, International Flame Research Foundation 9th Members' Conference, Noordwijkerhout, May 1989), suggested that using oxygen in place of air for combustion would also increase the kinetics. Oxy-fuel combustion, when flame temperature is controlled by burner design, further reduces thermal NOx formation by substantially eliminating N2 in combustion air. In both cases the net result is that the gas temperature in the first stage is increased, resulting in reduced NOx formation. Further, using both air preheat and oxy-fuel firing allows the first stage to be more deeply staged without degrading the flame stability. This allows even further reductions in NOx formation.
Oxy-fuel firing offers a further advantage for LNB's. Timothy et al (“Characteristics of Single Particle Coal Combustion”, 19th Symposium (international) on Combustion, The Combustion Institute, 1983) showed that devolatilization times are significantly reduced, and the volatile yield is increased, when coal is burned in oxygen enriched conditions. These tests were single particle combustion tests performed under highly fuel lean conditions, which does not provide information on how much oxygen is needed to accomplish this under more realistic combustion conditions. The higher volatile yield means that the combustibles in the gas phase increase as compared to the baseline—leading to a more fuel rich gas phase which inhibits NOx formation from the volatile nitrogen species. In addition, the fuel volatiles ignite rapidly and anchor the flame to the burner, which has been shown to lower NOx formation. The enhanced volatile yield also leads to shorter burnout times since less char is remaining.
Although the prior art describes several elegant enhancements for staged combustion and LNB's, several practical problems have limited their application. First, preheating the combustion air to the levels required to enhance the kinetics requires several modifications to both the system and the air piping. The air heater and economizer sections must be modified to allow the incoming air to be heated to higher temperatures, which may require modifications to the rest of the steam cycle components. The ductwork and windbox, as well as the burner itself, must also be modified to handle the hot air. All of the modifications can be costly and can have a negative impact on the operation of the boiler.
The primary barrier to the use of oxy-fuel firing in boilers has been the cost of oxygen. In order for the use of oxygen to be economic the fuel savings achieved by increasing the process efficiency must be greater than the cost of the supplied oxygen. For high temperature operations, such as furnaces without significant heat recovery, this is easily achieved. However, for more efficient operations, such as boilers, the fuel savings attainable by using oxy-fuel firing is typically much lower than the cost of oxygen. For example, if a typical coal-fired utility boiler were converted from air firing to oxygen firing, approximately 15 to 20% of the power output from that boiler would be required to produce the necessary oxygen. Clearly, this is uneconomic for most boilers.
Another potential problem of oxy-fuel firing or oxygen enrichment combustion in the boiler is the unbalancing of heat transfer in the boiler's radiative and convective sections. When oxygen enriched combustion is applied to a boiler furnace designed for air firing, the volume of flue gas is reduced and more heat becomes available and transferred in the radiant section and less heat is transferred in the convective section. Flue gas recirculation may become necessary in order to maintain proper heat transfer to various heat transfer sections of the boiler (i.e., water walls, super heater, reheater, and economizer, and feed water heater). However, handling recycled flue gas is very maintenance intensive and it is desirable to have an alternative clean diluent stream.
The control problem of NOx emissions from a coal fired boiler is further complicated as the firing conditions of the boiler change depending on the power output required for the steam turbine. Under boiler turndown conditions, minimum air flow requirements for the burners and coal mill have to be maintained and adequate mass flow through the boiler needs to be maintained to balance the steam production between the radiative (furnace) and convective sections of the boiler. To accomplish this the burners are typically run with excess air which is an inefficient operating condition and usually increases the overall stoichiometric ratio which often leads to higher NOx emissions per MMBtu fired. Although flue gas recirculation can be applied to reduce the excess air requirement, it complicates the boiler operation and raises the maintenance concerns.
Thus there remains a need for a method for achieving reduced NOx emissions in combustion of fuel (particularly coal) containing one or more nitrogenous compounds and especially for a method which can be carried out in existing furnaces without requiring extensive structural modifications or maintenance.